why AVR start permissive is 90% of the alternator Rated RPM

[Anand Gupta]

Why AVR start permissive is 90% of the alternator Rated RPM

what affects if permissive is less that than Rated RPM ?

 

[JohnRBaker]

This appears to have been posted in the wrong forum.

John R. Baker, P.E. (ret)
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[JedClampett]

How would we know?

 

[davidbeach]

Maybe there's a failure, if documented, that could make this the right home. But probably not.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[TugboatEng]

A failure to drop out below 90% rpm could possibly cause some problems, maybe even a disaster. Usually, AVRs start derating well before that.

 

[catserveng]

"Start Permissive" usually means the speed at which the AVR will go active on a startup. And is a feature found in several digital AVR regulators. But from the info you provided pretty much a guess from here.

If you could provide better details, like,
AVR make and model
Generator and prime mover info
Application info, like what kind of power system.

MikeL.

 

[Gr8blu]

The voltage regulator (AVR stands for "automatic" voltage regulator) can only regulate over a fairly narrow range: typically +/- 10 percent or so of a rated condition. A really good one might be able to regulate over a +/- 20 percent window. The regulator is going to try to regulate on the generator output VOLTAGE, which is proportional to both speed (at a fixed field current) and field current (at a fixed speed). To perform adequately, enough field current and speed have to be present to get the output voltage up to that +/- 10 percent window.

Secondly - the output frequency of the alternator is going to be proportional to the speed. Try to synchronize when too far off line frequency, and the system will just kick you off again. Leaving the regulator off until the alternator is up to at least 90 percent speed means the synchronization attempt won't occur until at least 95 percent speed - which is a rough, but reasonable, possibility.

Converting energy to motion for more than half a century

 

[Hoxton123]

In the dark days…

someone would want to start the generating set…

You would walk around the engine, look at all the guages, check everything was ok.

You would walk to the wall and start all the electrical auxiliaries.

Cooling water pump, radiator fans, building ventilation fans and so on.

Open the fuel valve; go to the air start and start the engine.

Afar it had settled at rated speed, 1500, 1000, 750 rpm in the civilised world (ouch!)

You walk around again and check everything, gauges, leaks etc.

Next you would move to the electrical control panel and turn the AVR control switch from off, to manual and then auto. You would check that the voltage was stable before synchronising the set to the mains

Now it’s all automated,

The sequence has not changed

 

[FreddyNurk]

There's a huge range of sizes and multiple excitation configurations that may apply. However, the summary is generally that if not close enough to rated speed, the AVR (which doesn't really care about frequency) will be trying to go flat out to bring the voltage up if not close enough to rated speed. That generally is not a good thing.

However, when coming up to speed, its entirely acceptable to enable excitation at some point below rated speed to bring the voltage up as well.

waross has made commentary here on what happens with smaller AVRs at much less than rated speed.
Worth considering, at least for smaller brushless systems, Under Frequency Rolloff (UFRO) does much the same thing below a certain speed, automatically. There are other subtle differences between a speed permissive and UFRO and what they can achieve, but both will not excite below a certain speed.

Excitation, particularly on smaller systems, needs to keep up at some point below rated speed so that in case of frequency dips due to block loading, the voltage doesn't disappear due to the AVR shutting off.

EDMS Australia

 

[RRaghunath]

Anand Gupta, Flux is proportional to V/f as you may be aware. That is lower the frequency, higher the flux for a given voltage.
When the AVR is switched on, it gets in to action and tries to maintain rated voltage at the generator terminals. Now, you can guess what would happen if the AVR is allowed to be ON at a low speed / frequency. The generator may get damaged due to overfluxing.
90% or 95% depends on generator overflux withstand capability. To my knowledge, it is 95% for large machines.